1. Field of the Invention
The invention relates to an optical amplifier and an optical communications network comprising said optical amplifier.
Optical communication, such as in particular telecommunication by means of optical glass fibres has become very well known. When an optical signal is transmitted through an optical fibre, losses occur which cause the signal to be weakened. In order to compensate for this weakening of the signal, the optical fibre or optical cable has to be provided with regularly spaced amplifiers. Such an amplifier can be based on a system in which first the optical signal is converted into an electrical signal by means of, for example, a photodiode, after which said signal is amplified and then converted again into an amplified optical signal, for example, by means of a laser.
In the past five years, the trend has been towards directly amplifying an optical signal by means of a so-called optical amplifier without using an intermediate electrical amplifier.
The invention more particularly relates to an optical amplifier for a weak optical signal, said amplifier comprising an optical amplification medium which is provided with a rare earth ion, first optical means for sending the optical signal through the amplification medium, a laser pump for exciting the rare earth ion, second optical means for sending the pump signal originating from the laser pump through the amplification medium, and third optical means for passing on the amplified optical signal originating from the amplification medium.
2. Description of the Related Art
Such an amplifier is for example known from Optical and Quantum Electronics 24 (1992), pp. 517-538.
The amplification medium of the known optical amplifier is a glass fibre which, as usual, comprises a core and a cladding of glass. A rare earth ion, notably Er.sup.3+, is dissolved in said core. Such a glass fibre is also referred to as optical fibre.
A disadvantage of the known optical amplifier is that the concentration of the rare earth ion, such as Er.sup.3+, in the core glass of the glass fibre must be very small because otherwise so-called concentration quenching would occur. This implies that in order to attain a reasonable degree of amplification a great length of the glass fibre amplification medium is required. For example, in Electronic Letters, Vol. 25, No. 24, pp. 1656-1657 (Nov. 1989) an Er.sup.3+ -doped glass fibre having a length of 100 m is mentioned. The concentration was 100 ppm Er.sup.3+. In this manner a gain in excess of 40 dB was attained.
When splitting optical fibres, there is a signal loss of approximately 3 dB. To compensate this signal loss, an optical glass fibre amplification medium of one or several meters is required. The price of such a glass fibre amplification medium is high and amounts to approximately 500-600 US$ per meter. For certain applications of optical systems this price is prohibitive.
In this context it is noted that Applicants have found that it is very desirable to apply optical communication, that is transmission via optical fibres, in in-home optical networks. These networks have a small transmission length in the range from a few meters to several hundred meters. However, said networks have many branches, resulting in a weakening of the signal which must be compensated by optical amplification. The price of the currently available amplifiers causes in-home networks to be commercially unfeasible.